Spectral and Spatial Variations of Flare Hard X-ray Footpoints

In a sample of strong RHESSI M-class flares we have made a study of the relationship between the `hardness' of the HXR spectrum and the intensity in the 30–50 keV energy range. In all events we find clear evidence for a `soft–hard–soft' pattern of correlation between hardness and flux, on time scales as short as 10 s. We investigate whether or not this pattern is intrinsic to the acceleration mechanism. The RHESSI images in this energy range are dominated by footpoint brightenings, and we have searched for a correlation between footpoint separation velocity and spectral hardness, to be compared qualitatively with theoretical flare models. We find quite systematic footpoint motions, and also note that episodes in which footpoint separation varies rapidly often correspond with episodes of significant change in the flare spectral index, though not as the simplest flare models would predict. We report also on one of our events, on 14 March 2002, which exhibits highly sheared HXR footpoint ribbons extending over a scale of 100 arc sec. For this flare we find a correlation between footpoint motion and hard X-ray flux. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022479610710